It is known in semiconductor production that the crystal defects in a semiconductor substrate and the metal contamination that occurs during the fabrication represent a serious factor in the deterioration of device characteristics. Known examples of such defects include white spots and dark currents in MOS (Metal Oxide Semiconductor), CCD (Charge Coupled Device), and CMOS (Complementary Metal Oxide Semiconductor) imaging sensors. Such deterioration of device characteristics is known to occur in RTS (Random Telegraph Signal noise), gate I-V characteristics, solar cell conversion efficiency, and laser output characteristics. One of the big challenges in semiconductor device production, then, is to reduce such defects and contamination in a semiconductor substrate.
There has been proposed a useful technique whereby CN− (cyano ions) present in a HCN solution are used to greatly reduce crystal defects and metal contamination in a semiconductor substrate (see, JP-A-10-74753, and JP-A-2005-33038). In this technique, the CN− are allowed to selectively react with the metals and crystal defects (dangling bonds) present on the semiconductor substrate surface or in the substrate. The reaction forms a stable complex and removes the metals, or forms bonds with the dangling bonds to passivate the defects. In this technique, the reaction sufficiently takes place at room temperature. It is known that the technique is very resistant to ultraviolet light or high temperatures of about 800° C.
Because the method using a HCN solution involves the high toxicity of HCN, it is difficult to use the method in the current production lines from the standpoint of chemical treatment. As a countermeasure, a method is proposed in which HCN is produced from methane and ammonia using a catalyst, and in which the HCN used is detoxified and decomposed into carbon dioxide gas and nitrogen by ultraviolet irradiation and ozone water treatment (see JP-A-2008-729).
The treatment method using HCN is considered very effective for the reduction of crystal defects and metal contamination. In fact, for example, there is a report of applying the method to silicon solar cells at the experimental level, and actually improving efficiency by a large margin (see O. Maida, A. Asano, M. Takahashi, H. Iwasa, H. Kobayashi, Surf. Sci. 542 (2003) 244).